We know Musk wants to go low-end and affordable - he's explicitly stated as such.
The question isn't whether or not Tesla will make a play for the low/mid-end market, it's whether or not they can win it.
The consumer car market is very different from the luxury car market. It's a world where it's not enough to execute well, but also do it with razor thin margins. Tesla has proven their ability to concoct grand ideas and bring them to life when money is no object - it remains to be seen if they can do it to the tune of $20-30K, or even lower.
Tackling the high end first was absolutely the correct strategy, but now the market is heating up. Are we so confident that Musk can lay waste to every single major automaker out there that people think they will be worth the entirety of Fiat, or half of GM?